Vapor electric discharge lamp



April 26, 1955 W. T. ANDERSON, JR

VAPOR ELECTRIC DISCHARGE LAMP Filed June 5, 1951 INVENTOR. l/V/L 4 MWZA'A/omm/a; we. BY

ATTOAMA'Y vAPoR ELECTRIC DISCHARGE LAMP William T. Anderson, Jr.,Maplewood, N. 3., assignor to Hanovia Chemical and Mfg. Company, Newark,N. 1., a corporation of New Jersey Application June 5, 1951, Serial No.229,971

3 Claims. (Cl. 313-184) The present invention deals with vapor electricdischarge lamps and particularly with vapor electric discharge lamps ofthe high pressure type.

A light source such as a tungsten filament incandescent lamp producesfull illumination almost immediately because the time required to heatthe filament to full brilliancy is less than one second.

Commercial fluorescent lamps operate with mercury vapor at very lowpressures, e. g. 0.015 millimeter, and require less than one-half minuteto reach full brightness.

Commercial high pressure lamps and superhigh pressure lamps require theattainment of mercury Vapor pressures from about 300 to about 60,000millimeters, depending on the type of lamp, before full brilliancy isreached. A time elapse of several minutes to about one-half hour may berequired for these lamps to reach full light output. Lamps of this typeemploy metals such as mercury, cadmium, sodium, caesium and the like forproviding the metal vapors in which during operation, the electricdischarge occurs. At room temperatures (25 centigrade) these metals aremostly either in a liquid or solid condition and have in the vapor phasepressures of less than about 0.002 millimeter on the mercury scale. As aresult, when such lamps are started there is a very appreciable timeperiod during which the metals must be evaporated and raised to highpressure vapor conditions.

During the beginning of this period the lamps produce very little usefulillumination.

The employment of mercury and a rare gas in the same lamp is not new.Rare atmospheric gases such as xenon, neon, argon and krypton at a fewto about 150 milliliters pressure have been used in mercury lamps forpurposes of starting the lamps and of providing the initial evaporationof the mercury. In these lamps the contribution of the rare gas to thelight ouput of the lamp is entirely negligible. During the first fewminutes of operation such lamps produce less than one percent of theirultimate average brightness. For some applications this behavior is notobjectionable, e. g. for street lighting and the like, but for otherapplications the initially low light output and brightness of theselamps makes them impractical.

Metal vapor lamps operate with an efficiency which cannot be approachedby filament type lamps and have a unit brightness greatly in excess ofother light sources. Hence, they are desired for very many applications.Their slowness in starting and their slow attainment of a usableintensity have limited their employment. Thus, these lamps might beexcellent for the lighting of airport runways, but the necessity to waitfor several minutes after the lamp has been started-for sufiicientusable illumination to be producedis most undesirable for such usage.

It is an object of the present invention to provide a high pressuremetal vapor discharge lamp capable of producing brilliant lightimmediately upon starting. It is another object of the present inventionto provide a high pressure or super high pressure metal vapor dischargelamp characterized by a comparatively short warm-up period. It is afurther object of the present invention to provide a high pressure metalvapor discharge lamp capable of application for many illuminationpurposes where existing high pressure type vapor lamps are notpractical. Other objects and advantages of this invention Will becomeapparent from the description hereinafter following and the drawingforming part hereof and which represents an elevational view of a lampembodying the subject matter of the present invention.

2,707,247 Patented Apr. 26, 1955 The present invention applies inparticular to lamps in which the final or operating vapor pressureexceeds about 10,000 millimeters and which have short discharge paths ofthe order of one centimeter and less. The ionizable filling of the lampaccording to this invention consists of a rare gas at a pressure between2000 and 11,000 millimeters at about 25 centigrade and a vaporizablemetal preferably in an amount sufiicient to become completely vaporizedduring the operation of the lamp. The lamp, therefore, is from themoment of start always a high pressure discharge lamp.

Although I may employ the rare gases argon, neon and krypton with avaporizable metal in accordance with the above and to certain advantage,I prefer to provide an electric arc in xenon at a pressure between 2,000and 11,000 millimeters, and to have a content of mercury evaporatecompletely during lamp operation as this will increase the arcstability. Instead of the highly diffuse discharge obtained With a lampare started in gas pressures less than 300 millimeters, I obtainimmediately a very concentrated and constricted arc of high brightness.I have found that if I provide a lamp with a high pressure gas fillingof xenon in addition to sufiicient mercury to provide the requiredmercury vapor pressure during operation of the device, I can obtain avery marked improvement in light production immediately upon thestarting of the lamp at 25 C. approximate ambient temperature and canachieve much faster warm-up of the lamp to the optimum mercury pressure.It is at the latter that full brilliancy of the arc is attained.

The xenon arc produces a brilliant light immediately and rapidly warmsthe lamp so that the mercury evaporates and is raised to high pressure,since xenon has an ionization potential of 12.08 volts and mercury anionization potential of 10.30 volts, the mercury is more rapidly ionizedthan the xenon. Consequently, as the mercury evaporates thecharacteristics of the lamp change. The white discharge of the xenon arcbecomes more bluish, the arc becomes more constricted with higher unitbrightness and the lamp efficiency is improved.

In order to obtain the high pressures of xenon and mercury required inmy invention, I estimate the capacity of the lamp envelope, e. g. inmilliliters, and for each milliliter of available space I provide acertain number of xenon molecules between about 5.4 10 and about 30 l0molecules per milliliter, and sufiicient mercury to provide in the vaporphase preferably about an equal number of mercury molecules, e. g. fromabout 4.3 l0 to 30 10 molecules per milliliter.

Xenon and mercury are monatomic gases and obey the gas laws of Boyle sothat the total pressure in the lamp envelope is the sum of the partialpressures of xenon and mercury.

A preferred type lamp to which my invention is applicable isillustrated.

Referring to the illustration, the lamp is a compact type lampcomprising either a substantially spherical or elliptical envelope 1 toprovide the required strength and made of fused quartz or a hightemperature glass, since it must withstand operating temperatures of theorder of 1200 Kelvin. The envelope 1 is provided with tubularextensions, e. g. tubes 2 and 3 projecting outwardly therefrompreferably oppositely of each other, which support a pair of spacerelectrodes 4 and 5, which are preferably tungsten electrodes in solid orcoiled form and spaced from each other. The other structures such as thereflecting members 6 and 7, the vacuum-tight seals 8 and 9, and theterminals 10 and 11, are merely illustratory for purposes of the presentinvention and are intended to show a type of lamp unit to which theessential embodiments of the invention are applicable.

The electrodes 4 and 5 are provided to supply electric power to the gasare which forms between them and are spaced from each other at adistance of the order of one centimeter and less. As illustrated theelectrodes are made of tungsten in the form of coils and are ofsubstantially equal size or mass which indicates that the lamp isdesigned for alternating current operation in which case the anodeelectrode would be about five-thirds the mass of the cathode electrodein order to withstand the intense negative ion bombardment.

The inner diameter normal to the arc of the envelope is approximatelyfive times the distance between the main electrodes 4 and 5 (arc-length)since this proportioning provides the greatest stability for the highpressure xenon are.

For example, the arc-length of a lamp according to this invention is 0.6centimeter and the inside diameter of the envelope normal to the arc isabout 3.0 centimeters. The envelope with electrodes assembled is degasedand evacuated. It is then provided with a filling consisting of 352milligrams of Xenon by weight, equivalent to l3.5 molecules of xenon permilliliter, and 300 milligrams of mercury by weight, equivalent to 8.210 molecules of mercury per milliliter. (The effective capacity of theenvelope was 12 milliliters.) At an average ambient temperature ofcentigrade, the xenon has a pressure of about 4,000 millimeters,slightly in excess of 5 atmospheres. The mercury has a vapor pressure ofonly 0.0018 millimeter, being mostly on the walls of the envelope inminute droplets.

The lamp is operated on 110 volt alternating current electric supplywith a ballast to limit the current to about 24 amperes. The dischargecan be initiated by a high voltage shock to ionize the gas. It can beassisted if de sired by radium emanations. At the instant of start ahighly concentrated are forms in the xenon. The concentration isgreatest if the lamp is supported so that the arc is mostly in thevertical position. The gas is heated rapidly and increases in pressureso that about 3 seconds after start the xenon gas pressure has changedfrom 4,000 millimeters to about 16,000 millimeters, corresponding to anaverage gas temperature of about 1300 Kelvin. At this time the averagebrightness of the arc is about 26,000 lamberts.

A lamp made in accordance with my invention is brighter after 3 secondsstart than any obtainable tungsten filament projection lamp. A 1,000Watt tungsten projection lamp 26 lumens per watt has an averagebrightness of 7,500 larnberts. Thus a lamp made in accordance with myinvention can successfully compete with incandescent lamps for manyillumination applications Where existing mercury lamps are initially toolow in brightness to be even considered.

I have described a lamp for operation at one kilowatt electric input. Myinvention applies equally well to lamps of other capacities. I have madelamps for 150, 500, and 700 watts to cite a few examples. I have usedxenon pressures to 11,000 millimeters at 25 C. as well as those in thevicinity of 3,000 millimeters pressure. I have operated lamps atcombined pressures in excess of 100 atmospheres. In every case myinvention has enabled me to operate lamps at an initially usefulbrightness.

Krypton gas can be used in place of xenon gas, but is not as effective.It has an ionization potential of 13.93 volts compared to 12.08 forxenon and 10.30 for mercury so that the ionization of krypton in thepresence of mercury is rapidly quenched by the latter. It krypton isused, the initial brightness is only about one-third of that obtained byxenon and there is subsequent decrease in brightness While the mercuryis evaporating due, possibly, to a quenching action. With xenon andmercury there is a negligible quenching action.

My invention is not limited to mercury. Other metals evaporate even moreslowly than mercury. However,

because of the quenching action, it seems desirable to use mercury inconjunction with the xenon and the second metal. Thus cadmium has anionization potential of 8.96 volts. At high pressures of severalatmospheres cadmium will completely quench the xenon and substantiallyquench the mercury. Unless mercury is used in combination, the initiallyhigh brightness obtained by the high pressure xenon arc will decrease asthe cadmium evaporates and the final brightness will be less than theinitial. The combination of cadmium and mercury can result in a lamp ofhigher brightness than the xenon alone with the additional advantagethat the mercury light emission is color-corrected by the addition ofthe cadmium green and red. In making such a lamp I provide the xenon gaspressure in the manner described and provide a combination of mercuryand cadmium such that the final operating pressure is about 50 per centthe partial pres sure of xenon, about 40 per cent the partial pressureof mercury, and about 10 per cent the partial pressure of cadmium.

What I claim is:

l. A vapor electric discharge lamp comprising an envelope of lighttransmissive high temperature glass having a substantially sphericalshape, a pair of electrodes spaced apart approximately one-fifth theinner diameter of the envelope and less than one centimeter, saidenvelope containing said electrodes and a filling of xenon gas at apressure in excess of 2000 millimeters and less than 11,000 millimetersat 25 C. and mercury between 4.3 10 and 30x10 molecules per milliliterof envelope volume. V

2. A vapor electric discharge lamp comprising a substantially sphericalfused quartz envelope containing a pair of electrodes spaced apartapproximately one-fifth the inner diameter of the envelope and less thanone centimeter, said diameter being normal to the lamp arc, and a gasfilling of xenon between about 5.4 1 0 and 30 10 molecules permilliliter of envelope volume, and mercury between 4.3 l0 and 30 10molecules per milliliter of envelope volume.

3. A vapor electric discharge lamp comprising a substantially sphericalfused quartz envelope containing a pair of electrodes spaced apart aboutone-fifth the inner diameter of the envelope and less than onecentimeter, one of said electrodes being an anode and one a cathode,said anode electrode having a mass approximately five-thirds that ofsaid cathode, a gas filling of xenon between 5.4 10 and 30 10 moleculesper milliliter of envelope volume and mercury between 4.3 10 and 30 10molecules per milliliter of envelope volume.

References Cited in the file of this patent UNITED STATES PATENTS NoelJan. 18,

1. A VAPOR ELECTRIC DISCHARGE LAMP COMPRISING AN ENVELOPE OF LIGHTTRANSMISSIVE HIGH TEMPERATURE GLASS HAVING A SUBSTANTIALLY SPHERICALSHAPE, A PAIR OF ELECTRODES SPACED APART APPROXIMATELY ONE-FIFTH THEINNER DIAMETER OF THE ENVELOPE AND LESS THAN ONE CENTIMETER, SAIDENVELOPE CONTAINING SAID ELECTRODES AND A FILLING OF XENON GAS AT APRESSURE IN EXCESS OF 2000 MILLIMETERS AND LESS THAN 11,000 MILLIMETERSAT 25* C. AND MERCURY BETWEEN 4.3X1019 AND 30X1019 MOLECULES PERMILLILITER OF ENVELOPE VOLUME.